Dear colleagues,

As far as the celtic languages and their names in french are concerned, I
have been in relation with the university of Brest (Pr Abalain).
Here is his proposition :

for manx, mannois is indeed the correct name, but we can also use manx in
french.
fo irish, irlandais is the best name, but we can also use gaélique
d'Irlande when it is necessary to stress taht it is a gaelic language
for the other languages, there are no problems : 
breton, cornique, gallois, gaélique d'Ecosse

As far as scots is concerned, the best name would be scots, as "écossais"
couls be congusing with scotish gaelic.

Best regards

Jean-Arthur Creff